Load-switching circuits using identical antiparallel or inverse-pair thyristor networks connected to respective taps of a power transformer and selectively connectable with a load so that the switching of the effective pair of antiparallel thyristors controls the load current, have been provided heretofore and represent effective full wave control devices.
Generally speaking a pair of identical inversely poled or antiparallel thyristors are connected in series with a choke between a tap of a power transformer and a load terminal and each of these antiparallel thyristor networks is provided with a commutation capacitor in series with a damping impedance which is connected across the network constituted by the choke and the antiparallel pair of thyristors.
Such circuitry can be operated to provide effective load switching, i.e. shifting of the load from electrical connection to one of the taps to electrical connection to the other of the taps.
Experience has shown that such circuits are subject to voltage transients, i.e. shock or surge voltages, which are applied to the blocked thyristors.
Since thyristors capable of withstanding high peak surges are expensive to manufacture and most thyristor components are not capable of withstanding such high peak surges, it is desirable to minimize the surge voltages to which the thyristors may be subjected.
The usual way of limiting the surge voltages for parallel-connected circuitry elements, e.g. varistors, cannot be used for thyristors because of the differences in the characteristics of these circuit elements. In the case of varistors, this characteristic is such that as the voltage sharply increases (during a surge potential) the varistor responds to limit further build up. With the thyristor, however, a high value of du/dt, i.e. rate of increase in the voltage or the time derivative of the voltage, can trigger a thyristor without application of a triggering pulse to the gate thereby detrimentally affecting the circuit and operation.